1. Field of the Invention
The present invention relates to a speaker system having a horn or an acoustic pipe provided in front of the speaker diaphragm and adapted for guiding sonic waves therefrom.
2. Description of the Prior Art
A speaker system is known in which a sound wave generated by a diaphragm is introduced to the second outlet opening of the speaker through a horn or an acoustic pipe provided on the front side of the diaphragm. This type of speaker systems is finding increasingly wide use because it provides a higher level of the output sound pressure and superior directivity as compared with ordinary speaker systems which do not have such a horn or acoustic pipe.
A description will be given hereinunder, with reference to the drawings, as to a known speaker system of the type having a horn or an acoustic pipe.
Referring to FIG. 9 which is a sectional view of a known speaker system of the type mentioned above, a back cavity 2 is provided on the rear side of a speaker unit 1 for the purpose of preventing radiation of reflected sound from the speaker diaphragm. A horn 9 is provided in front of the speaker diaphragm and extends towards the sound outlet opening of the speaker system. The cross-sectional area of the horn 9 is progressively increased from the end adjacent to the speaker diaphragm towards the end adjacent to the sound outlet opening of the speaker system. The horn 9 thus constitutes an acoustic path which introduces the sound wave output from the speaker. The change in the acoustic impedance at the sound outlet opening of the speaker system is made extremely small provided that the horn 9 has a length which is sufficiently greater than the length of the wavelengths of sound wave of the reproduction band. In such a case, a very good matching is obtained at the sound outlet opening of the speaker system so that a flat reproduction sound pressure frequency characteristic is obtained thus realizing an ideal speaker system. Actually, however, in case of incorporation of the speaker system in an acoustic apparatus in sever, it is not practically possible to design horn 9 having a length so large that it is sufficiently greater than the wavelengths of sound waves in the reproduction band. Therefore, the speaker system employing such horns usually exhibit reproduction sound pressure frequency characteristics which contain many peaks and troughs as shown in FIGS. 2 (graph B) and 8 (graph B).
This is attributable to the fact that reflection waves are generated at the sound outlet opening of the speaker due to a drastic change in the acoustic impedance. In consequence, resonances are caused in the acoustic path. The same problem is encountered also with a speaker system which makes use of an acoustic pipe in place of the horn 9. Thus, the speaker systems which employ acoustic pipes as the acoustic paths exhibit reproduction sound pressure frequency characteristics which contain many peaks and troughs. This is attributed to the fact that, as shown in FIG. 10, a resonance takes place at a frequency f which is represented by the following formula: EQU f=(2n-1)C/4L(n=1, 2, 3, . . . ,)
where, L represents the length of the acoustic pipe, while C represents the velocity of the sonic wave.
FIG. 10 illustrates the sound pressure distribution and velocity distribution as obtained when the number n is 2 (n=2).